In the plating of dielectric substrates by chemical (electroless) plating it is well known that suitable catalytic pretreatment is a prerequisite for effective electroless metal deposition. Such practices are well known and accepted in the art.
In examining the prior art for catalytic pretreatment it appears that while different procedures have been used, the incorporation of precious metals (e.g. palladium containing solutions) was common to all procedures. One catalytic system of particular interest is the two step process as disclosed in U.S. Pat. No. 3,011,920. In that process, a colloidal solution composed of tin (II) and precious metal salts, generally with hydrochloric acid, is used. The effective catalyst is proposed to be a colloid of an elemental precious metal (e.g. palladium) stabilized by the excess stannous chloride present in the media. While the system disclosed in U.S. Pat. No. 3,011,920 has been quite popular in commercial practices, rising costs of precious metals and miscellaneous product reliability problems has led to the quest for new systems in which the use of precious metals, tin, as well as of the hydrochloric acid would be completely eliminated.
In meeting this objective it was found, as described in U.S. Ser. No. 607,506 now U.S. 3,993,799, filed by the applicant herein, that colloidal systems based upon non-precious metals could constitute the basis for new commercial plating processes. More specifically, it was found that colloidal hydrous-oxides of non-precious metals (preferably selected from the group consisting of copper, cobalt and nickel) may be used in the direct replacement of the tin/palladium colloid followed by a treatment in a suitable reducing media. In the reducing media, reduction of the cationic portion of adduct derived through the adsorption from the colloidal media takes place, resulting in nucleating sites capable of initiation of the electroless process.
Another system utilizing non-precious metal ions is described in U.S. Pat. Nos. 3,772,056 and 3,772,078 to Polichette et al. In the systems described therein, the dielectric substrate to be plated is immersed in a solution of non-precious metal ions, and then dried to promote adherence of the ions to the dielectric surface. The retained ions may then be reduced to the metallic state by heat or chemical means. A system of this nature is a significant departure from the standard commercial process as a result of the required intermediate drying step to achieve the required adhesion. In standard commercial processes, the article to be plated is passed from one aqueous treating tank to another with intermediate rinsing steps to remove excessive material and hence minimize cross-contamination. A procedure of this nature is essential due to the automation of most plating operations and thus the Polichette et al procedure would require a major modification in standard equipment and procedures, thus rendering the process unattractive and costly. A process for electroless plating, to be commercially feasible, should be adaptable to existing commercial processing techniques.
The catalytic compositions described herein are compatible with existing commercial processing tecnhiques and apparatus and also eliminate the necessity of precious metals, tin and acid.